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Subject: Re: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets
Sure, it's fine to separate it out into another patch and optimize on top later on. Though the previous comment of avoiding repeatedly computing sg_num in datapath still holds: set sg_num only once in probe() time then, in which case the sg_num needed can be simply inferred from dev->max_mtu rather than dev->mtu.
On 8/9/2022 7:56 AM, Si-Wei Liu wrote:
External email: Use caution opening links or attachmentsa guest VM driver changing mtu to smaller one is valid use case. However, current optimization suggested in the patch doesn't degrade any performance. Performing close() and open() sequence is good idea, that I would like to take up next after this patch as its going to be more than one patch to achieve it.
On 8/8/2022 12:31 AM, Gavin Li wrote:
Nope, for e.g. "ip link dev eth0 set mtu 1500" can be done from guest on a virtio-net device with 9000 MTU (as defined in guest xml). Basically
On 8/6/2022 6:11 AM, Si-Wei Liu wrote:
External email: Use caution opening links or attachmentsACK
On 8/1/2022 9:45 PM, Gavin Li wrote:
Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for bigGUEST GSO (virtio term), or GRO HW (netdev core term) it should have
packets even when GUEST_* offloads are not present on the device.
However, if GSO is not supported,
been be called.
ACK
it would be sufficient to allocateDitto.
segments to cover just up the MTU size and no further. Allocating the
maximum amount of segments results in a large waste of buffer space in
the queue, which limits the number of packets that can be buffered and
can result in reduced performance.
Therefore, if GSO is not supported,
ACK
use the MTU to calculate theDIV_ROUND_UP() can be used?
optimal amount of segments required.
Below is the iperf TCP test results over a Mellanox NIC, using vDPA for
1 VQ, queue size 1024, before and after the change, with the iperf
server running over the virtio-net interface.
MTU(Bytes)/Bandwidth (Gbit/s)
Before After
1500 22.5 22.4
9000 12.8 25.9
Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
---
drivers/net/virtio_net.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ec8e1b3108c3..d36918c1809d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -222,6 +222,9 @@ struct virtnet_info {
/* I like... big packets and I cannot lie! */
bool big_packets;
+ /* Indicates GSO support */
+ bool gso_is_supported;
+
/* Host will merge rx buffers for big packets (shake it! shake
it!) */
bool mergeable_rx_bufs;
@@ -1312,14 +1315,21 @@ static int add_recvbuf_small(struct
virtnet_info *vi, struct receive_queue *rq,
static int add_recvbuf_big(struct virtnet_info *vi, struct
receive_queue *rq,
gfp_t gfp)
{
+ unsigned int sg_num = MAX_SKB_FRAGS;
struct page *first, *list = NULL;
char *p;
int i, err, offset;
- sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
+ if (!vi->gso_is_supported) {
+ unsigned int mtu = vi->dev->mtu;
+
+ sg_num = (mtu % PAGE_SIZE) ? mtu / PAGE_SIZE + 1 : mtu
/ PAGE_SIZE;
Not sure how to do it and align it with align with new mtu during
Since this branch slightly adds up cost to the datapath, I wonder if
this sg_num can be saved and set only once (generally in virtnet_probe
time) in struct virtnet_info?
.ndo_change_mtu()---as you mentioned in the following mail. Any idea?
ndo_change_mtu might be in vendor specific code and unmanageable. In
my case, the mtu can only be changed in the xml of the guest vm.
guest user can set MTU to any valid value lower than the original
HOST_MTU. In the vendor defined .ndo_change_mtu() op, dev_validate_mtu()
should have validated the MTU value before coming down to it. And I
suspect you might want to do virtnet_close() and virtnet_open()
before/after changing the buffer size on the fly (the netif_running()
case), implementing .ndo_change_mtu() will be needed anyway.
I think that it should be fine, though you may want to deal with the XDPACK+ }Comment doesn't match code.
+
+ sg_init_table(rq->sg, sg_num + 2);
/* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */
ACK. The two calls virtnet_set_guest_offloads and- for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {Please do the same for virtnet_clear_guest_offloads(), and
+ for (i = sg_num + 1; i > 1; --i) {
first = get_a_page(rq, gfp);
if (!first) {
if (list)
@@ -1350,7 +1360,7 @@ static int add_recvbuf_big(struct virtnet_info
*vi, struct receive_queue *rq,
/* chain first in list head */
first->private = (unsigned long)list;
- err = virtqueue_add_inbuf(rq->vq, rq->sg, MAX_SKB_FRAGS + 2,
+ err = virtqueue_add_inbuf(rq->vq, rq->sg, sg_num + 2,
first, gfp);
if (err < 0)
give_pages(rq, first);
@@ -3571,8 +3581,10 @@ static int virtnet_probe(struct virtio_device
*vdev)
if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6) ||
virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_ECN) ||
- virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UFO))
+ virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UFO)) {
vi->big_packets = true;
+ vi->gso_is_supported = true;
correspondingly virtnet_restore_guest_offloads() as well. Not sure why
virtnet_clear_guest_offloads() or the caller doesn't unset big_packet on
successful return, seems like a bug to me.
virtnet_set_guest_offloads is also called by virtnet_set_features. Do
you think if I can do this in virtnet_set_guest_offloads?
path not to regress it.
-Siwei
Thanks,
-Siwei
+ }
if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF))
vi->mergeable_rx_bufs = true;
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